Invisible Decays of the $Z$ in Portal Matter Models at Tera-Z

Abstract

Searches for new contributions to the invisible width of the $Z$, $ΔΓ_{inv}$, can be used to provide model-dependent constraints on new physics that may arise from an unknown dark sector. In the vector portal setup considered here it is shown that loops of portal matter (PM) fields, having both Standard Model (SM) and dark sector quantum numbers, can lead to new contributions to $ΔΓ_{inv}$ that satisfy current experimental constraints while being simultaneously accessible to measurements at the future Tera-Z. Such PM fields, which here take the form of vector-like fermions, are well known to be able to induce kinetic mixing (KM) between the SM and dark photons (DP) at low energies. This analysis is performed within a simplified version of a previously examined `$E_6-$like' setup that predicts a calculable value for the strength of KM in a phenomenologically interesting range. On the other hand, owing to the constraint on the invisible branching fraction of the Higgs, the tree-level mixing of the SM and a weak isosinglet dark Higgs fields yields a value of $ΔΓ_{inv}$ which is far too small to be observable. A similar result holds for the shift in the $Z$'s invisible width induced by the mass and/or KM of the $Z$ with the light DP which then subsequently decays to dark matter. On the other hand, PM models wherein the light dark Higgs carries a significant admixture of a $T_3\neq 0$ weak isospin component are shown to already violate existing $ΔΓ_{inv}$ constraints in the absence of parameter tuning. Future measurements of $ΔΓ_{inv}$ can thus play an important role in providing indirect constraints on dark sector interactions and PM model building, in particular, on PM loops not necessarily associated with the generation of KM of the DP with the corresponding SM field.